Elsevier

Acta Materialia

Volume 51, Issue 12, 16 July 2003, Pages 3429-3443
Acta Materialia

Deformation behavior of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass over a wide range of strain-rates and temperatures

https://doi.org/10.1016/S1359-6454(03)00164-2Get rights and content

Abstract

The stress-strain relations for the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass (Vitreloy 1) over a broad range of temperatures (room temperature to its supercooled liquid region) and strain rates (10−5 to 103 s−1) were established in uniaxial compression using both quasi-static and dynamic Kolsky (split Hopkinson) pressure bar loading systems. Relaxation and jump in strain rate experiments were conducted to further understand the time dependent behavior of Vitreloy 1. The material exhibited superplastic flow above its glass transition temperature (623 K) and strain rates of up to 1 s−1. The viscosity in the homogeneous deformation regime was found to decrease dramatically with increasing strain rate. A fictive stress model was used to describe the basic deformation features of Vitreloy 1 under constant strain-rate loading as well as multiple strain-rate loading at high temperatures.

Introduction

Progress made in the last few decades has shown that metastable glassy metals can be formed in binary, ternary and multi-component alloy systems (e.g., [1], [2]). Atoms of different metals can be kinetically constrained or frozen by rapid quenching techniques such that no long-range order exists in the as processed alloy systems. Following the discovery of a binary Au-Si amorphous alloy in form of thin ribbon using very high cooling rate of 105–106 K/s [3], the early search for bulk glass forming alloys dated back to the 70s, when several ternary metallic glass formers were found such as Pd-Ni-P [4] and Pd-Cu-Si [5], with dimensions in the millimeter scale. The late 80s and early 90s witnessed a revolution in the development of bulk metallic glasses with lower cooling rates resulting in larger sizes and thus making them attractive candidates for many structural applications [6], [7], [8], [9], [10].

Among these new bulk glass forming metallic alloys, a Zr-based material, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy 1), exhibits high resistance with respect to crystallization in its wide supercooled liquid region. It is processed at low cooling rate of around 1 K/s [7] and can be cast up in sizes of up to 2 to 4 cm in thickness. The high yield stress [10], [11] and the high strength to density ratio of this bulk metallic glass makes the material an excellent candidate for structural applications. Research on its viscosity, relaxation, kinetics and crystallization [12], [13], [14], [15] has shown that thermo-mechanical characterization up to its crystallization temperature is made possible by the increased thermal stability of the glassy alloy with respect to crystallization.

Generally speaking, the deformation of metallic glasses can be classified into two modes, namely, homogeneous and inhomogeneous deformation. In spite of their metallic bonding, all the metallic glasses discovered so far exhibit shear localization at room temperature, leading to catastrophic shear failure immediately following yield. At higher temperatures, they can be deformed homogeneously, exhibiting considerable amount of inelastic deformation. Recently, homogeneous deformation of several metallic glasses around their glass transition temperature was investigated, including the tensile deformation of 20 μm thick Zr65Al10Ni10Cu15 ribbons [16] and 40 μm thick La55Al25Ni20 ribbons [17]. The use of ribbons instead of their bulk forms in millimeter scale was probably due to limitations in the availability of materials and the experimental setup. In these investigations, necking appears during most tensile tests, making the observation of variation of the steady state stress difficult. Experiments on bulk Zr55Al10Ni5Cu30 specimens of 3 mm in diameter [18] and bulk Pd40Ni10Cu30P20 specimens of 2 mm in diameter [19] have been performed in their homogeneous deformation regime. The strain rate was limited to the quasi-static range and the temperature in the vicinity of the corresponding glass transition temperature. Recently, Nieh et al. [20] investigated the plasticity and localization of the bulk amorphous Zr52.5Al10Ti5Cu17.9Ni14.6 alloy in its supercooled region. They attributed the non-Newtonian behavior observed during deformation to the formation of nanocrystallites during the high temperature deformation.

There exist a number of theories to describe the deformation behavior of metallic glasses. Argon developed a model based on the idealization of two deformation modes, namely the diffuse shear transformation and the dislocation loop formation, to analyze the boundary between homogeneous and inhomogeneous flow of Pd-Si metallic glass [21], [22], [23]. Spaepen proposed a theory based on the free volume created by external stress and its annihilation by diffusion [24]. This was further modified by Steif et al. to analyze mechanical deformation problems by including additional free volume change due to pressure [25]. Khonik suggested a directional structural relaxation model, which states that each rearrangement event can be interpreted as a thermally activated shear due to local atomic structures and subsequently nearly athermal viscous flow by external stress [26]. Among these models, Speapen’s free volume based model seems to be the most widely cited to interpret the deformation behavior of metallic glasses. Duine used the free volume based viscosity assumption by Speapen and defect evolution assumption to analyze the kinetic process of defects and their steady state concentration [27]. de Hey et al. applied the same idea to temperature induced structural evolution of some metallic glasses, leading to the conclusion that additional free volume is created as compared with thermal equilibrium due to plastic deformation [28], [29], [30]. Recently, Kato and Chen et al. adopted a simple model based on the concept of fictive stress to simulate stress-strain behavior of metallic glasses [31], [32], which will be discussed in detail later.

Deformation of bulk metallic glass differs from that of a crystalline metallic material due to the absence of long-range order, i.e., amorphous in nature. Relatively little is known regarding the effect of rate and temperature on the deformation behavior of Vitreloy 1, particularly in the supercooled liquid region. The objective of this study is to systematically explore the thermo-mechanical behavior of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy 1) bulk metallic glass under uniaxial compression subject to a wide range of strain rates and temperatures. Compression loading was chosen in the present study because tensile loading typically induces necking at relatively small strains while compression can result in uniform deformation up to large strains provided care is used in conducting the experiments. The experimental results can aid in the development of appropriate constitutive models and delineate possible deformation modes for bulk metallic glasses over such a wide range of temperatures and strain rates. Such constitutive models are expected to provide design rules for processing and structural applications of bulk metallic glasses. In addition, they may be also employed in the design of a new generation of high-pressure casting facilities and net-shape forming thermomechanical processes for structural amorphous metals.

Section snippets

Experimental

The bulk metallic glass used in this investigation is Zr41.25Ti13.75Cu12.5Ni10Be22.5 with the commercial name Vitreloy 1 (commonly referred to as Vit 1). Vitreloy 1 is the trademark (™) name used by Liquid Metal Technologies, Inc., Laguna Niguel, CA, for the afore mentioned bulk metallic glass. The processing, physical and mechanical properties of this material are well documented in the literature (see for example, [7], [10], [11], [12], [13], [15], [33], [34], [35], [36]). Relevant

Effect of temperature on stress-strain behavior

The influence of temperature on failure modes is one of the key features of deformation of materials and their characterization, i.e., brittle to ductile transition. A typical example of the stress-strain curves is shown in Fig. 1 for a strain rate of 1.0×10−1s-1. To clearly distinguish each stress-strain curve while presenting all of them in the same plot, some of the curves were shifted along the strain axis by a strain value in the range of 0–0.02. As shown in Fig. 1, Vitreloy 1 exhibited a

Effects of strain rate and temperature on viscosity

At sufficiently low temperatures, metallic glass behaves like a typical brittle solid material. To process such a material, it is necessary to heat it to a temperature at which it either softens or melts to undergo a shape change operation. Thus, the ability to measure and characterize the viscosity of a metallic glass is important for optimizing its processing conditions. Among all fluids, a Newtonian fluid has the simplest constitutive behavior, in which the strain rate is directly

Modeling

In this section, a phenomenological model is employed to describe the flow behavior of Vitreloy 1 in the homogeneous regime. Recently, a model was proposed using the concept of fictive stress [19] in which the uniaxial stress rate is described using a simple Maxwell model,dσdt=Edεdtσλ,where E is the Young’s modulus which is a function of temperature, ε is the strain, σ is the stress and λ is the relaxation time. If the relaxation time λ is only a function of temperature, then for a given

Conclusions

The uniaxial stress-strain behavior of a bulk metallic glass, namely Vitreloy 1 (Zr41.2Ti13.8Cu12.5Ni10Be22.5) was investigated experimentally over a wide range of strain rates as well as temperatures. The following conclusions are drawn from this investigation:

  • 1.

    The deformation of Vitreloy 1 bulk metallic glass is very sensitive to loading rate and temperature near and above its glass transition, 623 K, i.e., in the supercooled liquid region. Quasi-static experiments revealed that Vitreloy 1

Acknowledgements

This work was sponsored by the Structural Amorphous Metals Program of the Defense Advanced Research Projects Agency (DARPA), under ARO Contract No. DAAD19-01-1-0525, and, in part by the Center for Science and Engineering of Materials at the California Institute of Technology through a grant from the MRSEC program of the National Science Foundation, which are gratefully acknowledged.

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